1. Field of Invention
The present invention relates to laser, and more particularly to a third harmonic laser system which is capable of generating high third harmonic power with high efficiency.
2. Description of Related Arts
Light Amplification by Simulated Emission of Radiation (Laser) has been widespread all over the world. Thanks to its extremely high brightness and coherency, laser has been found extensive applications in situations which require rapidly delivering of huge amount of energy to a desire position without affecting the nearby objects. Typical examples of utilizing laser for practical purposes include material cutting, especially for cutting extremely hard materials and for the cuttings that require high precision and quality, medical treatment of cancer, and machining of high precision Printed Circuit Board (PCB) etc.
Solid-state laser is highly monochromatic by its nature. Thus, for some specific reasons or extension of its applications, a fundamental laser beam emitted by a high intensity source may have to be raised to second or third harmonic of its frequency, for examples U.S. Pat. Nos. 6,115,402, 5,835,513, 5,742,626, 5,144,630, and China Patent Nos. 130,311 and 1,058,740. In these examples, optical arrangements, each of which usually comprises at least a non-linear medium, are used for generating nth harmonic laser of the frequency of fundamental laser beam. The third harmonic laser is produced by mixing a second harmonic laser and a fundamental laser through the nonlinear medium, such as crystal, in an external laser cavity.
However, such arrangements for raising the frequency of the fundamental laser beam to higher harmonic orders, such as its third harmonic frequency, may fail. In worse, the optical arrangement may even produce unacceptably incoherent or poor quality laser, thus wasting generally expensive equipment and raw material. The reasons for these problems are as follows:
First, since the production of fundamental laser beam depends on spontaneous emission of photon which is essentially a random process, and thus the emission efficiency of the fundamental laser source itself is usually as low as 20% in practice. On the other hand, the conversion efficiency of the emitted fundamental laser beam to the second harmonic laser beam is proportional to the square of the power of the fundamental laser beam. As a result, the overall conversion efficiency is still low.
Second, in order to achieve desirable intensity of the laser beam, in practice, a plurality of optical components is used to converge laser beams so as to obtain higher laser intensity. However, since each laser beam carries a huge amount of energy by itself, the laser beam may even damage the optical components. Once there are defects formed on the lenses, the quality of the resulting converged laser beam will be deteriorated.
Third, mixing the second harmonic laser beam with the fundamental laser beam through the nonlinear medium in the external laser cavity to generate the third harmonic laser beam is usually performed by a single pass tripling process. This leads to huge wastage of uncombined second harmonic and fundamental laser beams.
China Patent No. 1,288,275 utilizes a second harmonic laser multiple mixing chamber to enhance the conversion efficiency of the second harmonic laser beam. However, since the fundamental laser beam produced, as cited in that China patent, is unidirectionally outputted from a fundamental wave resonator, the conversion efficiency of the second harmonic laser beam, though higher, is still far from satisfaction.
Therefore, intra-cavity third harmonic laser generation has been introduced, such as U.S. Pat. Nos. 6,241,720, 6,229,829, 6,002,695, and 5,898,717, and China Patent Nos. 2,351,897 and 1,285,636. In these patents, the third harmonic laser beam is produced by mixing fundamental laser wave with double-pass second harmonic beam once. These techniques are known as single-pass tripling.
However, a significant disadvantage of these single-pass tripling is still high wastage of unconverted second harmonic laser beam since the fundamental laser beam and the second harmonic laser beam only pass through the non-linear medium once.
U.S. Pat. Nos. 5,936,983, and 5,943,351 suggest generating third harmonic laser beam by multi-pass tripling process, i.e. generating third harmonic laser beam by mixing second harmonic laser beam and fundamental laser beam through a non-linear crystal for more than once. Although, through the multi-pass tripling process, the conversion efficiency is enhanced as compared to conventional single pass tripling process, the multi-pass tripling process must generate a number of single-pass tripling third harmonic laser beams which need being reflected to combine into a single outgoing high intensity third harmonic laser beam. These reflections may make the resulting third harmonic laser beam slightly not overlap with each other. As a consequence, there may be more than one third harmonic laser beam emitting from the third harmonic laser system simultaneously. Then, by using such laser for, say, precision drilling, some undesirable holes may be appeared.
Furthermore, it is a natural consequence that the multi-pass tripling process will ultimately impose greater burden to optical components involved as compared with single-pass tripling because the number of times the laser beam passes through the optical components are greater. In view of the above, a high efficiency and high quality third harmonic laser beam therefore is highly demanded.
A main object of the present invention is to provide a third harmonic laser system which is capable of producing efficient and high power third harmonic laser beam.
Another object of the present invention is to provide a third harmonic laser system which substantially increases the overall conversion efficiency as compared with conventional third harmonic laser system, so as to provide highly efficient third harmonic laser beam.
Another object of the present invention is to provide a third harmonic laser system which is capable of producing single and coherent third harmonic laser beam so as to enhance the quality of outputted laser.
Another object of the present invention is to provide a third harmonic laser system which is capable of producing efficient and high intensity third harmonic laser beam without inducing any significant damage to the optical components, especially non-linear crystals, of the laser system. As a result, the laser system can produce high quality third harmonic laser beam.
Accordingly, in order to accomplish the above objects, the present invention provides a third harmonic laser system, comprising:
a fundamental wave resonator which comprises a first laser active medium for generating at least a fundamental laser beam from a front end thereof, wherein the fundamental laser beam has a predetermined fundamental wavelength;
a second harmonic laser generator, which is optically communicated with the fundamental wave resonator and comprises:
a first polarizing mirror, which is highly reflective to the fundamental laser beam, being in optical communication with the fundamental wave resonator and orientated in such a manner that the first polarizing mirror is capable of reflecting the fundamental laser beam with vertical linear polarization generated by the fundamental wave resonator;
a dual wavelength wave-plate which is capable of rotating a portion of second harmonic laser beam with horizontal polarization direction into vertically polarized direction laser beam and allowing an unconverted portion of the horizontally polarized fundamental laser beam to pass through; and
a first type-I nonlinear crystal which is optically communicated with the first polarizing mirror and the dual wavelength wave-plate in such a manner that the first type-I nonlinear crystal is capable of converting the fundamental laser beam with vertically polarized direction and reflected from the first polarizing mirror into a horizontally polarized harmonic laser beam which is half of that of the fundamental wavelength while allowing an unconverted portion of the fundamental laser beam to pass through; and
a third harmonic laser generator, which is in optical communication with the second harmonic generator and comprises:
a second type-I nonlinear crystal which is capable of mixing and converting a predetermined portion of the fundamental laser beam and the second harmonic laser beam respectively into a horizontally polarized third harmonic laser beam having a third harmonic wavelength which is one-third of the fundamental wavelength, while allowing unconverted fundamental laser beam and second harmonic laser beam to pass through;
a second polarizing mirror, which is anti-reflective to the third harmonic laser beam and highly reflective to the second harmonic laser beam and the fundamental laser beam, optically transmitting horizontally polarized third harmonic laser beam generated by the second nonlinear crystal so as to reflect the second harmonic laser beam and the fundamental laser beam to the second harmonic laser generator; and
a third harmonic laser end mirror, which is highly reflective to the fundamental laser beam, the second harmonic laser beam, and the third harmonic laser beam, optically communicated with the second type-I nonlinear crystal for reflecting the fundamental laser beam, the second harmonic laser beam and the third harmonic laser beam to the second polarizing mirror via the second type-I nonlinear crystal, such that the third harmonic laser beam is capable of being outputted by passing through the second polarizing mirror.